The present invention relates to an improvement in a differential pressure transducer of a semiconductor strain gauge of cantilever type and, more particularly, to an improvement in the construction for fixing the cantilever.
In the differential pressure transducer making use of a cantilever, a strain gauge is attached to one or each surface of the cantilever by bonding, evaporation, soldering or welding. The cantilever is fixed at its one end to a base or a main body of the transducer while the other is left for free displacement. In operation, the amount of displacement corresponding to the pressure differential is detected as a change in the electric resistance of the strain gauge.
The differential pressure transducers are usually directly attached to the process pipings in various plants such as chemical plants, steel-making plants, petroleum plants and the like. Since the pressure differential transducer is intended for use at the site, it must stand for use at a large variety of temperatures of between, for example, -40.degree. C. and +120.degree. C. Also, the differential pressure transducer is directly subjected to environmental vibration. Thus, the differential pressure transducer is used under extremely severe conditions.
Conventionally, the fixing of the cantilever at its one end has been made by means of screws. The fixed end, however, is liable to be loosened due to a repeated change in the ambient temperature, because the material of the cantilever and the fixing members have different coefficients of thermal expansion. This loosening inconveniently results in a non-linearity of the characteristic of the differential pressure transducer and increase of the hysteresis. Further, the performance of the transducer is gradually deteriorated due to such changes a making the transducer unusable. This type of differential pressure transducer is disclosed in the specification of U.S. Pat. No. 3,363,456.
In addition, this type of differential pressure transducer requires a space for driving the screws by which the end of the cantilever is fixed, which in turn makes it difficult to design and a small-sized and light-weight differential pressure transducer. In this type of differential pressure transducer, the joints between the cantilever and the fixing member, between the fixing member and the hermetic member and between the hermetic member and the main body of the transducer are made by welding. The welding between the hermetic member and the main body, however, is liable to result in separation or cracks due to the thermal distorsion of the hermetic member.